Skiving of cylindrical gears

ABSTRACT

A skiving tool comprising a cutter head ( 2 ) having a plurality of cutter blade mounting and positioning slots ( 8 ) arranged spaced, preferably equidistant, about the periphery ( 7 ) of the cutter head with the blade slots, and hence the cutting blades ( 4 ), preferably oriented perpendicular to the axis of rotation (A) of the cutter head. Alternatively, the blade slots may be inclined from the perpendicular orientation by less than 50 degrees, preferably less than 20 degrees, thereby forming a conical shaped cutter. Additionally, the blade slots may be positioned to extend radially from the cutter head axis whereby the longitudinal axis of a cutter blade will intersect the cutter head axis, or the blade slots may be radially offset from the cutter head axis. The blade slots may have any cross-sectional shape such as square, rectangular or those types having generally V-shaped seating surfaces ( 10 ) comprising a pair of angled mounting surfaces ( 12, 14 ) each less than 90 degrees. In contrast to known cutting blade configurations, the cutting blade ( 4 ) of the present invention has its cutting face ( 16 ) formed in a surface of the cutting blade that is located opposite to the seating surface or V-shaped seating surfaces ( 13, 15 ) of the cutting blade.

FIELD OF THE INVENTION

The invention is directed to cutting of cylindrical gears and inparticular to cutting such gears by skiving.

BACKGROUND OF THE INVENTION

Skiving of cylindrical gears (also known as “hob peeling”) is a cuttingprocess that has existed for many years, primarily for manufacturinginternal ring gears (e.g. see DE 243514). Like honing, skiving uses therelative sliding motion between two “cylindrical gears” whose axes areinclined. A skiving cutter usually looks like a shaping cutter with ahelix angle, for example 20°, different than the helix angle of thecylindrical gear to be machined (e.g. US 2011/0268523). Other skivingtools comprise bar- or stick-shaped cutting blades arranged in a cutterhead according to a hyperboloid as shown in, for example, US2012/0282055.

Due to the continuous chip removal in skiving, the process is multipletimes faster than shaping and more flexible than broaching, but itpresents a challenge to machines and tools. While the roll motionbetween the cutting edges and the gear tooth slots occurs with themachine spindle RPM, the relative axial cutting motion is generally onlyabout one third of the circumferential speed of the cutter. The cuttingcomponents of rolling and cutting which result in a “spiral peeling” arerepresented by the process designation skiving.

Because of the relatively low dynamic stiffness in the gear trains ofmechanical machines as well as the fast wear of uncoated cutters,skiving of cylindrical gears failed to achieve a breakthrough againstshaping or hobbing until recently. The latest machine tools with directdrive train and stiff electronic gear boxes present an optimal basis forthe skiving process. Complex tool geometry and the latest coatingtechnology have contributed to give the soft skiving of cylindricalgears a recent breakthrough.

SUMMARY OF THE INVENTION

The invention is directed to a skiving tool comprising a cutter headhaving a plurality of cutter blade mounting and positioning slots (bladeslots) arranged spaced, preferably equidistant, about the periphery ofthe cutter head with the blade slots, and hence the cutting blades,preferably oriented perpendicular to the axis of rotation of the cutterhead. Alternatively, the blade slots may be inclined from theperpendicular orientation by less than 50 degrees, preferably less than20 degrees, thereby forming a conical shaped cutter. Additionally, theblade slots may be positioned to extend radially from the cutter headaxis whereby the longitudinal axis of a cutter blade will intersect thecutter head axis, or the blade slots may be radially offset from thecutter head axis. The blade slots may have any cross-sectional shapesuch as square, rectangular or those types having generally V-shapedseating surfaces comprising a pair of angled mounting surfaces each lessthan 90 degrees. In contrast to known cutting blade configurations, thecutting blade of the present invention has its cutting face formed in asurface of the cutting blade that is located opposite to the seatingsurface or V-shaped seating surfaces of the cutting blade.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the basic geometry and kinematic of skiving.

FIG. 2 shows pitch cylinders of workpiece and tool.

FIG. 3 illustrates calculation of cutting velocity.

FIG. 4 depicts a procedure for the calculation of the tool referenceprofile.

FIG. 5 illustrates calculation of machine settings.

FIG. 6 shows coated solid high speed steel skiving cutters.

FIG. 7 illustrates cutting velocity calculation.

FIGS. 8(a)-8(d) show a stick blade skiving cutter with carbide cuttingblades.

FIG. 9 is a Table for determining cutter diameter and number of cuttingblades.

FIG. 10 illustrates virtual tool teeth on a skiving cutter.

FIG. 11 is a comparison of productivity for shaping, hobbing andskiving.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The terms “invention,” “the invention,” and “the present invention” usedin this specification are intended to refer broadly to all of thesubject matter of this specification and any patent claims below.Statements containing these terms should not be understood to limit thesubject matter described herein or to limit the meaning or scope of anypatent claims below. Furthermore, this specification does not seek todescribe or limit the subject matter covered by any claims in anyparticular part, paragraph, statement or drawing of the application. Thesubject matter should be understood by reference to the entirespecification, all drawings and any claim below. The invention iscapable of other constructions and of being practiced or being carriedout in various ways. Also, it is understood that the phraseology andterminology used herein is for the purposes of description and shouldnot be regarded as limiting.

The details of the invention will now be discussed with reference to theaccompanying drawings which illustrate the invention by way of exampleonly. In the drawings, similar features or components will be referredto by like reference numbers. For a better understanding of theinvention and ease of viewing, doors and any internal or externalguarding have been omitted from the drawings.

The use of “including”, “having” and “comprising” and variations thereofherein is meant to encompass the items listed thereafter and equivalentsthereof as well as additional items. The use of letters to identifyelements of a method or process is simply for identification and is notmeant to indicate that the elements should be performed in a particularorder.

Although references may be made below to directions such as upper,lower, upward, downward, rearward, bottom, top, front, rear, etc., indescribing the drawings, there references are made relative to thedrawings (as normally viewed) for convenience. These directions are notintended to be taken literally or limit the present invention in anyform. In addition, terms such as “first”, “second”, “third”, etc., areused to herein for purposes of description and are not intended toindicate or imply importance or significance unless otherwise specified.

The geometric setup of a skiving cutter relative to an internal ringgear is shown in FIG. 1. The front view onto the generating gear systemis shown in the upper left graphic. The ring gear is oriented in themain coordinate system with its axis of rotation collinear to theY-axis. The cutter center (origin of Rw) is positioned out of the centerof Y₄ in the X₄-Z₄ plane by a radial distance vector Ex. The pitchcircles of the cutter and the ring gear contact tangentially at thelowest point of the pitch circle. The top view which shows the toolinclination angle or shaft angle Σ is drawn below the front view. Incase of a spur gear the stroke motion is directed in line with theY-axis. The relative velocity required as cutting motion is generatedwith a shaft angle Σ around the X₄-axis of the coordinate system shownin FIG. 1. In case of a helical gear, the cutter inclination can bechosen independently from the helix angle. However, a helix angle of 20°or larger offers the possibility to match it with the shaft angle Σ anduse a simplified spur gear style shaper cutter for the skivingoperation. Also in this case, the stroke motion is oriented in Ydirection but an incremental rotation ω₂ which depends on the strokefeed has to be added to ω₁. The shaft angle Σ can also be defineddifferently than the helix angle which still will require the sameincremental ω₂, but the tool front face orientation and side reliefangles have to be calculated from the difference between helix angle andthe shaft angle Σ. The side view to the right in FIG. 1 shows a secondpossible tool inclination which is called the tilt angle. This tool tiltangle can be used to increase the effective relief angles between theblades and the slots and it can also be utilized to eliminateinterferences between the back side of a long spur gear style shapercutter with minimum relief angles. Within limits, it is also possible toutilize the tilt angle for pressure angle corrections.

The three dimensional side view in FIG. 2 shows an internal helical gearwith a shaft angle Σ between work and tool. FIG. 2 shows the baseangular velocities of the work ω₁ and the formula for its calculation.FIG. 2 also includes the incremental angular velocity ω₂ and the formulato calculate it from the helix angle and the axial feed motion (strokemotion). The cutting velocity is calculated as the difference vectorbetween the circumferential velocity vectors of work and tool in thecutting zone. FIG. 3 shows a top view of the configuration between tooland work with the velocity vectors.

The reference profile of the tool is determined from the referenceprofile of the work applying the procedure shown in FIG. 4. Thereference profile of the work with its pressure angles α₁ and α₂ and itspoint width Wp is drawn as a trapezoidal channel which is cut with aplane under the shaft angle Σ (FIG. 4, top, right side). The profilewhich is defined by the intersecting lines between plane and channelrepresents the reference profile of the tool. This tool referenceprofile is used in order to generate the involute in the tool cuttingfront (FIG. 4, bottom right side).

The machine setting calculation is shown in FIG. 5 on the example of abevel gear cutting machine, for example, a machine such as disclosed inU.S. Pat. No. 6,712,566. The explanation of the formula symbols are:

X-Y-Z . . . Machine axis directions (Y is perpendicular to the drawingplane)

Σ . . . Shaft angle between cutter and work

CRT . . . Cutter reference height

B . . . Cutter swing angle

P_(Z) . . . Pivot distance to spindle front in Z-direction if B=0°

P_(X) . . . Pivot distance to spindle center line in X-direction if B=0°

Z₁, Z₂ . . . Components in Z-direction

Depending on the helix directions in work and cutter, the cutting takesplace below or above the work gear center line in order to keep theB-axis angle below 90°. In case of no corrections, the crossing pointbetween the cutter axis and the work axis lies in the cutter referenceplane.

Traditionally, as mentioned above, skiving is performed with typicalgear shaper cutters. A variety of different tools used for skiving isshown in FIG. 6. The first cutter (left) is a shaft type which isslightly tapered without helix angle in the cutting teeth. This cuttercan be used for gears with a helix angle. The shaft angle between cutterand work will be set to the helix angle of the work. This also meansthat the helix angle of the work should be above 10° in order togenerate sufficient cutting speed. Due to the straight nature of thecutting teeth, work pieces with small diameter and large face widthmight cause interferences between the slot and the far end of thecutting blade. The skiving cutter in the center of FIG. 6 is awafer-type cutter (preferably with TiN coating) which can bere-sharpened a few times. The cutting teeth are also straight, whichmakes this cutter also only suitable for work pieces with a helix angle.The wafer cutter has very short relieved teeth, which will preventinterference problems in case of helical slots that wind around a smalldiameter work piece. The skiving cutter to the right of FIG. 6 hasserrated blade front faces and teeth which are oriented under a helixangle. Preferably, the skiving cutter of FIG. 6, right, is coated with aTiAlN coating.

If the helix angle of the work piece is 15° and the tool helix angle is20°, then the shaft angle between skiving cutter and work has to besetup to 5° (same helix direction). If the helix directions are oppositethen a shaft angle of 35° has to be used. An interesting case occurs ifthe gear helix angle of the work is identical to the cutter helix angle(same amount and same hand). In this case the shaft angle between cutterand work is zero and no skiving motion is generated. The calculation ofthe cutting surface speed depending on the helix angle β of the work andthe shaft angle Σ is shown in FIG. 7. The upper graphic represents theunrolled pitch cylinder with teeth and slots indicated (see also rightside graphic in FIG. 7) for a spur gear. With β=0 the formula issimplified to the first special case. The lower graphic shows theformula simplification for the second special case, which occurs if thehelix angle β is equal the shaft angle Σ. The cutting velocity formulaconsiders next to the circumferential velocity at the work gear pitchdiameter the helix angle β of the work and the shaft angle Σ betweenwork and skiving cutter. The cutting velocity vector is automaticallydirected in the flank lead direction if the formula in FIG. 7 isapplied. Although the formula indicates some interplay between Σ and β,the major parameter for generation of sufficient cutting velocity is theshaft angle Σ between the work and tool axes.

A cutter head 2 which uses stick blades 4 has been developed especiallyfor skiving (see FIG. 8a-8c ). The blade material is preferably carbideand the blade profiles are 3-face ground and all-around coatedpreferably with TiAlN (titanium aluminum nitride) coating although othercoatings are not excluded. The blade profile 6 resembles an involutewhich is derived from the tool reference profile in FIG. 4. The bladescan either be ground as full profile blades just like the profiles ofthe cutters shown in FIG. 6, or as alternating left flank-right flankblades which allows it to realize sufficient side rake angles. Thealternate blade arrangement offers very good tool life and anexceptionally smooth cutting operation. However, the productivity isslightly lower than the one using full profile blades.

The preferred cutter of the present invention is illustrated in FIG. 8cand comprises a cutter head 2 having a front face 3 and a back face 5with a plurality of cutter blade mounting and positioning slots (bladeslots) 8 arranged spaced, preferably equidistant, about the periphery 7of the cutter head. The blade slots 8, and hence the cutting blades 4,are preferably oriented perpendicular to the axis of rotation, A, of thecutter head but may be inclined from the perpendicular by less than 50degrees, preferably less than 20 degrees, thereby forming a conicalshaped cutter. Additionally, the blade slots may be positioned to extendradially from the cutter head axis whereby the longitudinal axis of acutter blade will intersect the cutter head axis A (FIG. 10), or theblade slots may be radially offset from the cutter head axis. The bladeslots may have any cross-sectional shape such as square, rectangular orthose types having generally V-shaped seating surfaces 10 comprising apair of angled mounting surfaces 12, 14 each less than 90 degrees (e.g.see U.S. Pat. No. 5,890,946).

However, in contrast to the known cutting blade configuration, thecutting blade of the present invention has its cutting face 16 formed ina surface of the cutting blade 4 that is located opposite of the seatingsurfaces 13, 15 of the cutting blade (see blade cross-sectional viewFIG. 8d ) and, hence, also opposite of the V-shaped seating surfaces 12,14 of cutter head 2 when the cutting blade 4 is mounted in the cutterhead. The V-shaped seating surfaces 12, 14 open-up in a direction towardthe front face 3 and the cutting faces 16 of the cutting blades 4 areoriented generally toward the front face 3. With this arrangement,forces encountered during cutting are transmitted to the V-shapedseating surfaces 12, 14 thereby fortifying the seating of the cuttingblades 4 in the cutter head 2 resulting in a cutting tool of enhancedstability during the cutting process. In FIG. 8d , any side rake andrelief angles have been omitted for simplicity of viewing and angle K(see FIG. 8c also) is the cutting face angle of blade 4 which isgenerally equal to the shaft angle Σ.

Due to the design of the cutter head of FIGS. 8a-8c , the blades havespaces between them which are larger than the tooth thickness of thereference profile. The cutters may be configured for a certain module ofgear, such that the blades in the cutter head represent every second,third or fourth slot of the reference profile. Regarding a low workpiece runout and high spacing quality, it is preferable to avoid acommon denominator between the theoretical number of skiving cutterteeth and the number of work gear teeth. The same rule applies of coursefor solid skiving cutters as well.

A procedure was developed which allows a minimum of different cutterhead types for a variety of gears. For example, external gears withoutpitch diameter limitation or internal ring gears with a minimal pitchdiameter of about 330 mm and above can be skived with a 9 inch (228.6mm) diameter peripheral cutter head. The Table in FIG. 9 uses modulesfrom 2 to 7 mm in 0.5 mm steps to indicate an initial tool pitchdiameter, D₀₂. The effective tool module, m_(tool), is calculated fromthe equation shown in FIG. 4 to account for the angular orientation(e.g. 20°) of the Tool Rotation Plane (i.e. shaft angle Σ) and fromthere the theoretical number of tool teeth Z₂ is determined byD₀₂/m_(tool). The tool teeth number Z₂ must be rounded up or down (byone or more integer values) in order to arrive at an integer number ofteeth (Selected Z₂) from which an effective pitch diameter of the tool,(Selected Z₂) X m_(tool)=D₀₂*, is determined. The developed stick bladesystem allows adjustment of the blade stick in or out by some smallamount to match the required pitch diameter for the number of teethselected.

However, the preferred cutter heads within the 9 inch size family ofcutters usually comprise blade slot numbers of 15, 17, 19, 21 or 23. Inthe next columns of the Table, all existing integer fractions between 2and 5 are determined (the range may be expanded to 6, 7, 8 or more). Thegoal is to find the largest number of slots which is available in the 9inch diameter line of cutters to assure the maximal productivity. Inother words, the skiving cutter never represents the theoretical tooltooth number with the number of slots but only a fraction thereof. Thetheoretical number of tool teeth becomes the virtual tool tooth numberof which only a fraction is represented on the cutter head. If a numberis selected and typed in the spread sheet, next to the actual fractionof slot and theoretical tooth number the resulting number of cutterslots is shown in the last column. If this number does not match anexisting cutter head, then a second or third number has to be chosenuntil a matching cutter is found. In some cases (e.g. Modules 3, 5, 5.5and 7 of FIG. 9), the immediate up and/or down rounded integer may notprovide an acceptable number of cutter slots and so the rounding up anddown is continued until a suitable number of cutter slots can beattained. It is to be understood that while the foregoing example isbased upon a 9 inch diameter cutter, other cutter head diameters, e.g.4.5 inches (114.3 mm), 7 inches (177.8 mm) and 8 inches (203.2 mm), arealso contemplated by the invention.

Depending on the number of teeth of the work gear, the virtual number oftool teeth may be even and never is a prime number. This will not be ofany disadvantage, as long as a hunting tooth relationship between workand virtual cutter is given. In such cases, also between work and realcutter, the hunting tooth principle exists. The peripheral stick bladecutter design will physically not allow the virtual number of blades tofit next to each other. The cutter of FIG. 10 represents each othertooth which is indicated with the dashed drawn (virtual) blades betweenthe real blades.

Solid cutters made from high speed steel (HSS), such as M48, with aTiAlN coating are generally suitable for a surface speed of up to about100 m/min in a wet skiving environment. Carbide stick blades (e.g. 10%Co-90% WC) with a TiAlN coating allow about 300 m/min surface speedwhich may be considered to be a “critical speed”. However, with respectto tool wear, it should be considered that in skiving, very high toolrotational speeds are required in order to achieve the desired surfacespeed. This in turn creates a profile sliding which is superimposed onthe cutting speed. The profile sliding has its highest value at theblade tip which also undergoes a long chip removing engagement path andthereby resulting in the blade tip being vulnerable to additional siderelief wear.

Cutting trials have shown that in skiving it is possible to reduce thesurface speed to between 150 and 200 m/min which is below the criticalspeed (i.e. Under-Critical Speed, UCS) while increasing the infeed andfeed rate in order to keep the productivity high, yet receive asignificant improvement in tool life.

As an example, a module 4.0 mm gear was cut by wet UCS skiving withTiAlN coated carbide blades and 172 m/min surface speed. A firstroughing pass used an infeed setting of 5 mm and a feed rate of 0.045 mmper blade. The chips were large and only slightly curved. Each chiprepresents one flank and part of the tooth slot bottom. A second passused an infeed setting of 3 mm and a feed rate of 0.28 mm per blade.This chip consisted of two flanks connected by a bottom portion. Afinishing pass used an infeed of 1.00 mm and a feed rate of 0.015 mm perblade. This chip also had two flanks and a bottom chip portion connectedto a form U-shaped appearance.

In another example, the same type of gear was cut utilizing TiAlN coatedcarbide blades and 172 m/min surface speed in a three-pass dry cuttingprocess using the same infeed values and feed rates as applied in thewet cutting. Except for a color change due process heat, the dry chipsgenerally have the same appearance as the chips from wet cutting.

The comparison between the wet and dry skiving processes with coatedcarbide stick blades shows the dry process as being advantageous. Theprocess heat helps to plastically deform the chip during the shearingaction. If the process parameters and tool geometry are chosen to movethe process heat into the chips and then away from tool and work piecewith the chips, a cool skiving process is the result. Dry skivingdelivers a better surface finish and causes equal or even lesser toolwear than the “wet” process version. Additionally, the chip surface onthe side adjacent to the sheared off side is smoother and machine powerreadings showed about 15% lower spindle power during dry skiving. Thecurrent skiving developments indicate that dry skiving delivers a bettertool life, which is anticipated to be even more significant than in drycutting of bevel gears.

Dry UCS skiving with coated carbide blades results in the optimalcombination between low tool wear and low skiving times. An additionaladvantage is that machines with medium speed high-torque spindles (e.g.1000 RPM maximum for machine size 600 mm Outside Diameter) can beapplied without compromising the performance of the machine e.g. forbevel gears which require low RPM and high torque. This advantage isimportant if a manufacturer performs skiving on bevel gear cuttingmachines. The manufacturer expects appropriate machine performance forall bevel gear machining but also appreciates the ability to practice anefficient skiving process on the same machine.

Thermographic imaging of dry skiving in the above example taken duringthe roughing pass at mid face reveals the highest temperature of 107° F.(42° C.) occurs around the cutter and at the work gear sections whichmoves away from the cut. This is also the temperature that can bemeasured on part and cutter after the cycle. It can be concluded thatdry UCS process has an optimal heat transfer into the chips and quicklyreaches a rather low steady state temperature of cutter and workholding.

A productivity comparison between the traditional processes hobbing andshaping and three variations of the skiving process is shown in FIG. 11.In order to establish the same basis for each process, an external ringgear with the gear data as those in the above examples was used for allprocesses. The objective was a finishing quality with scallop orgenerating flat amplitudes at or below 5 μm. The shaping process used aTiAlN coated shaper cutter from HSS material with 34 teeth and was setupas a 3-cut finishing cycle. The identical shaper cutter was used for thewet skiving where the cutting was done in four passes. For the hobbingprocess, a one-start hob with 16 gashes also from TiAlN coated HSSmaterial was utilized in a 2-cut cycle. Dry skiving with TiAlN coatedH10F carbide blades is represented in the diagram as UCS-skiving with172 m/min and as high speed skiving with 300 m/min, both setup as athree pass cycle. The dry skiving bars are based on a 24 blade and 9″diameter cutter head. The chip thickness in the case of UCS-skiving is10% to 20% higher than in the case of high speed skiving which reducesthe productivity difference between the two process variations and yetgives the UCS-skiving a tool life advantage. FIG. 11 indicates thatskiving has between 6 to 12 times the productivity of shaping andbetween 1.6 and 3.3 times the productivity of hobbing.

While the invention has been described with reference to preferredembodiments it is to be understood that the invention is not limited tothe particulars thereof. The present invention is intended to includemodifications which would be apparent to those skilled in the art towhich the subject matter pertains without deviating from the spirit andscope of the appended claims.

What is claimed is:
 1. A cutting blade for skiving of gears, said cutting blade comprising: a bar-shape with a length extending in a longitudinal direction and comprising a cutting face located at one end of said length, said cutting blade further comprising a pair of angled mounting surfaces defining a generally V-shape extending along at least a portion of said length, said cutting face being arranged opposite to said pair of angled mounting surfaces whereby with respect to a longitudinal cross-section of said cutting blade, the angled mounting surfaces are spaced from and not in contact with the cutting face.
 2. In a method of producing a toothed workpiece by skiving, the improvement comprising: providing a tool comprising a cutter head, said cutter head being rotatable about an axis of rotation and comprising a front face, a back face, and a peripheral surface located between said front face and said back face, said peripheral surface comprising a plurality of cutting blade mounting and positioning slots arranged therein and extending in a longitudinal direction inward toward said cutter head axis, wherein each of said mounting and position slots includes seating surfaces defining a generally V-shape which open-up in a direction toward said front face, providing a cutting blade located in at least one of said plurality of cutting blade mounting and positioning slots, said cutting blade being bar-shaped with a length extending in a longitudinal direction and comprising a cutting face located at one end of said length with said cutting face projecting outward from said peripheral surface and being oriented in a direction toward said front face, said cutting blade further comprising a pair of angled mounting surfaces defining a generally V-shape extending along at least a portion of said length, said cutting face being arranged opposite to said pair of angled mounting surfaces, said pair of angled mounting surfaces being positioned in and complementary with said V-shape seating surfaces of said cutter head, engaging said tool and said workpiece, machining teeth on said workpiece by skiving.
 3. The method of claim 2 wherein said machining is carried out at a surface speed of 150-200 m/min.
 4. The method of claim 2 wherein said cutting blade comprises carbide.
 5. The method of claim 2 wherein said cutting blade is coated with TiAlN. 